The invention relates to a method and an apparatus for continuously filtering a polymeric melt.
A method and apparatus of the described type is generally known and described, for example, in the treatise "Filtration System Maintains Continuous Flow During Filter Changes," as published in SPE Journal, Vol. 27, No. 4, April 1971. In the apparatus disclosed in that publication, the melt flow is guided through a filter chamber which includes a filter element. In the event that the filter element becomes clogged and needs to be cleaned or exchanged, the entire melt flow is guided through a second, parallel filter chamber. This method has the disadvantage that the difference of the pressure drop between a cleaned and a clogged filter chamber becomes directly noticeable as a pressure fluctuation in the melt flow.
The known filtering apparatus also has the disadvantage that it is necessary to install twice the filtering capacity, so as to ensure a continuous filtration without interrupting the production. The entire melt flow is guided through a filter chamber with a filter element, which has the total required filtering capacity. The filtering capacity relates, in this instance, to the specific throughput or the maximum throughput of the filter elements related to the permissible pressure loss. The second filter chamber is in a standby position, and is not in use until the clogged filter element needs to be exchanged.
In the known filtering apparatus, the melt flow enters through an opening at the front end into the filter chamber, so that the melt is distributed over an annular space between the filter chamber and the filter element and flows through the filter element toward an outlet end. In so doing, the rate of flow of the melt in the annular space decreases steadily in the direction toward the front of the filter chamber opposite to the inlet end. Thus, at the closed end of the filter element the flow discontinues. The consequence is a long dwelling time of the melt in this region which causes the melt to decompose or crack.
The consequence of the cracking of the melt is that the polymer becomes thinner and, thus, is able to flow more easily through the filter medium. However, in the following process, these decomposed polymers lead to imperfections or damage of the end product. In addition, the flow conditions change, since the surfaces with higher specific filter loads clog faster.
Accordingly, it is the object of the present invention to further develop a method and a filtering apparatus of the initially described type, so that the filtering capacity installed as a whole insignificantly exceeds the need of filtering capacity for a continuous filtration of a melt flow, there being produced a melt flow which is filtered as uniformly as possible without substantial pressure fluctuations and without decomposed polymer components.
Furthermore, it is an object to provide a continuous melt filtering apparatus wherein, even with little utilization of the filtering capacity or with a lower specific load on the filtering surface, the rate of the flow does not fall below a minimum that is justifiable at any point.